Saffil Automotive

Exhaust System Backgrounds

New emission regulations being put in place throughout the world are aimed at reducing the gaseous and particulate emissions into the atmosphere. These regulatory drives demand the need to improve technology as they are constantly made more rigorous.

The market is moving to meet these demands by:

•	The introduction of more efficient thinner walled catalytic converters. This is driving force for Ecoflex® non-intumescent mat systems to enable thin wall substrates to be supported and canned without risk of damaging the monolith.
•	Moving the catalytic converter close to the engine (close-coupled or pre-converter). This is the driving force for higher performance materials to withstand high operating temperatures (up to 1050°C) and high vibration forces when the converter is connected directly to the manifold. Ecoflex® mats deliver high temperature performance giving outstanding durability in close-coupled converters (CCC) applications.
•	Introduction of Diesel Emission Control Systems to remove both the particulate (through Diesel Particulate Filters – DPF) and NOx reduction technology (SCR) from diesel engines.

Design Challenges
When designing a catalytic converter the aim is to meet the emissions regulations set by the various governing bodies whilst retaining engine performance.

Key considerations include:

•	Engine type (i.e. Gasoline or Diesel)
•	Converter location (i.e. close coupled, toe board, under body)
•	Converter Size and Cell/Wall Configuration
•	Available space
•	Hot face / shell requirements
•	Acceptable pressure drop (power loss) across the system

Catalytic Converters & Diesel Particulate Filters
A catalytic converter helps control harmful emissions from engine sources by converting the hydrocarbons (HC), nitrogen oxides and carbon monoxide (CO) in the engine's exhaust into carbon dioxide (CO₂), nitrogen (N₂) and water (H₂O) vapour.

The essential components of a catalytic converter are as follows:
Substrate
A ceramic honeycomb structure (typically cordierite) made up of mostly square channels, to which the exhaust gas can flow through and over the precious metal catalyst. The channels provide a large surface area for the application of wash-coat and precious-metal catalyst to convert the engine exhaust to less harmful components.

The application trend in ceramic substrates is to increase the efficiency of the converter by manufacturing the internal walls thinner and increasing the number of channels / cells (cell density).
The advantage of thin / ultra thin wall substrates are a greater geometric surface area (active surface area) for catalytic conversion so less catalyst can be used or you can have a more efficient converter to meet future regulations. However, thinner wall substrates are also much more fragile so there is in increased risk of damage during manufacture and operation with expanding support mats.

Washcoat
The washcoat is applied on to the ceramic substrate and contains the active catalyst (precious metals such as Platinum, Palladium, and Rhodium) and converts the exhaust gases into less harmful gases and water vapour.
Unique types of wash-coats and processes have been developed by catalysing specialists to help auto manufacturers reach specific goals, including: reducing the amount of precious metals loading required to meet emissions regulations, accommodating a variety of ceramic substrate material compositions and mitigating the effects of engine fluctuations.

Active Substrates
This type of honeycomb combines an extruded ceramic with a catalyst in one material. The current types have been developed preferred for heavy-duty applications. Due to their low isostatic strength similar design criteria can be applied as for ultra thin wall substrates.

Support Mat
The mat supports the ceramic catalysed substrate within the exhaust, and provides a holding force on the system to prevent substrate slippage at operating temperatures and vibration conditions. The mat also insulates the system to ensure the shell temperature meet the desired values. Mats are available typically in fibre only or intumescent format.

Shell / Can
The shell is the outer metal component that houses the catalytic converter and support mat and is integrated as part of the exhaust system. It is made out of steel; two steel types are predominantly used today, austhenitic and ferritic, depending on the system’s parameters.

For more details on the different exhaust gas aftertreatment technologies please see below